This is because research accelerators use UCx targets as sources of heavy ions, in particular at the GANIL (Grand Accélérateur National d'Ions Lourds [French National Heavy Ion Large Accelerator]) at Caen with the SPIRAL 2 (Système de Production d'Ions Radio-Actifs en Ligne de 2ème Génération [2nd Generation In-Line Radioactive Ion Production System]) facility.
The target material UCx, which can be used in the context of the operation of research accelerators, is conventionally synthesized by carbothermic reduction starting from a superstoichiometric mixture of graphite and of UO2 powder and then compressed to form centimeter-sized pellets. Its structural composition is mainly made up of two phases: a uranium dicarbide UC2 phase predominantly constituting the target material (at a level of 90% by weight) and another phase composed of free carbon, denoted CF, present in the graphitic form. In terms of distribution by volume, the latter, conventionally present at a level of 70%, can eventually occupy, in the final UCx material, values ranging from 0% to more than 75%. Very slight traces of UC can also be present in the initial material (typically less than 1%), which are synthesized locally during the carbothermic reduction stage.
It should be noted that the actual material to be stabilized, once irradiated, can include fission/activation products, such as Co, Cs, B, Br, Kr, Zr, Rh, and the like.
Generally, the stabilization methods should make it possible to respond to the following constraints:
the conversion of the UCx material into the form of a stabilized product of UOx type (U3O8, UO2, UO3, and the like) has to be compatible with the requirements of the outlets/storage areas envisaged by the nuclear safety authorities and ANDRA, the French national agency for the management of radioactive waste;
the application of a process for the stabilization of the UCx materials via a specific oxidative heat treatment must make it possible to control the oxidation reaction, which is highly exothermic, and to banish any phenomenon of uncontrolled runaway during the reaction;
the control, by a parametric and bounded range, of the chemical reactivity of the material (limitation of thermal runaway, selectivity of the oxidation reaction, control of the ignition temperature) during the process for the oxidative treatment of the UCx materials, in order to prevent any erratic operation. FIG. 1 thus illustrates the sudden and uncontrolled recovery in reactivity and more specifically an example of thermal runaway characterized by a pseudoperiodical overheating during the oxidation of a sample of uranium metal at 390° C. (Yves Adda, Etude cinétique de l'oxydation de la nitruration et de l'hydruration de I'uranium [Kinetic study of the oxidation, nitridation and hydridation of uranium], French Atomic Energy Commission Report No. 757, (1958);
the possibility of minimizing the production of gaseous discharges and of effluents, always expensive and restrictive for the environment of nuclear technology, by the use of an optimum operating range for the process which makes it possible to completely and solely stabilize the UC2 phase while prohibiting the oxidation with the excess free carbon present in the UCx material. The eventual objective is to make use of a process in a nuclear environment (shielded cell) by a simple treatment method which does not generate liquid effluents;
the confirmation of the absence of reactivity of the products once the latter are stabilized in the oxidized form, the final material having to be stable with regard to the reactivity with the air and under ambient temperature and pressure conditions;
the use of a stabilization process compatible with semi-industrial operating requirements: reduced treatment time, robustness of the process, notably with regard to the variability in the input (weight of material, density, porosity, phases) and controlling monitoring indicators throughout the process.
Currently, UCx targets which have already been used are stored in the expectation of a suitable outlet and/or of a treatment process; this is the case, for example, in the ISOLDE (Isotope Separator On Line Detector) facility at Geneva.
Chemical reprocessing methods have already been described, notably in the international patent application: WO/2004/012206, which presents a process for electrochemical oxidation by the dissolution route. The treatment proposed renders it completely incompatible with the UCx material targeted as the application of this process generates a considerable amount of liquid effluents (resulting from a chemical dissolution) not corresponding to the objectives desired in the present invention.
There also exist scientific publications relating to the oxidation of uranium-comprising carbides of UC/UC2 type which can be categorized chiefly into three main families according to the nature of the oxidant employed: carbon dioxide, liquid water or water in the vapor form, and molecular oxygen, at different concentrations.
As regards the oxidation reactions of actinide carbides with CO2, the authors Peakall, K. A. and Antill J. E., Oxydation of Uranium Monocarbide, J. Less-Common Metals, 4 (1961), 426-435, record oxidation studies carried out on UC under an atmosphere of carbon dioxide as oxidizing gas. The results obtained mention that the reactivity of the carbides with CO2 is relatively slow and incompatible with the objective of providing an industrial process (notably with regard to the treatment time criteria). Murbach et al., E. W. and G. E. Brand, 1965, “Pyrochemical reprocessing of uranium carbide”, Summary Report, Atomics International, page 38, furthermore observed reactivities which are highly variable, as a function of the morphological nature of the UC, which result in unfinished and incomplete oxidation cycles, which is unacceptable for the targeted application. On the whole, these observations, relating to a significant decrease in the kinetics for the oxidation of carbides in the presence of CO2, are incompatible with the requirements imposed and mentioned above for the reprocessing of the material formed of UCx targets, which restrict in favor of a faster conversion.
As regards the reactions for the oxidation of actinide carbides with water in the liquid form and in the vapor form, several studies given below by way of example, including those mentioned in the following papers: Bradley, M., “Hydrolysis of Uranium Carbides between 25 and 100° C.”, II Uranium Dicarbide, Uranium Metal Monocarbide Mixtures and Uranium Monocarbide-Dicarbide Mixtures, Inorganic Chemistry, 3 (1964), 189-195, Herrmann, B. and Herrmann, F. J., Cinétique d'oxydation du mono carbure d'uranium par l'oxygène sec ou humide [Kinetics of oxidation of uranium monocarbide by dry or humid oxygen], French Atomic Energy Commission Report, 19 (1968), show that carbides react with water and water vapor. The results mention that the water vapor is an important vector of the oxidation mechanism and that the pre-exposure to air or to a weakly oxidizing humid atmosphere significantly increases their reactivity. It should be noted that the treatments for the oxidation of carbides with water in the liquid form are entirely unsuitable for the process envisaged with the material formed of UCx targets from the viewpoint of the major constraints related notably to the treatment of the effluents which this would subsequently generate. Although the presence of water vapor has the effect of increasing the reactivity of the carbides, notably hyperstoichiometrically, by a faster rate of conversion into the oxide phase, the oxidation studies presented in these papers under an anisothermal atmosphere and only in the presence of water vapor alone exhibit two major disadvantages for the definition of a process suited to the material based on uranium carbide which is the subject matter of the stabilization process of the present invention because:
of a slower conversion of the carbides into the oxide phase in the presence of water vapor alone and in the presence of molecular oxygen under similar oxidation conditions;
of the formation of new gaseous products, as described in Litz, M., Uranium Carbides: “Their Preparation, Structure and Hydrolysis”, PhD Thesis, Ohio State University, NP-1453 (1945): CH4 (for UC), C2H6 (for UC2) and in particular the production of molecular hydrogen H2 (whether this is from UC or from UC2), in potentially large amounts, the potentially explosive nature of which is highly damaging to the safety of the process. The result of this is that none of the results obtained in the presence of water vapor, predominantly on UC, is directly transposable to the requirement specified for the UCx material as a result of the constraints presented above and also the nature of the variability in the input (high content of excess carbon, which results in an additional increase in H2 by hydrolysis/gasification unless specific precautions are taken.
Finally, regarding the reactions for the oxidation of UC and of UC2 with O2, many studies have been published on the oxidation of uranium-comprising carbides under an atmosphere of molecular oxygen at different contents. Nevertheless, it should be pointed out that these studies, except for those of Nawada H. P. et al., Thermogravimetric study of the oxidation behaviour of uranium dicarbide, Journal of Thermal Analysis, 35 (1989), 1145-1155, relate to the UC material of stoichiometric composition and which is consequently substantially different in nature and behavior from the multiphase UCx material targeted by the present invention, the latter being composed of two main phases (of uranium carbide and of free carbon in the graphitic form). The only data available with regard to stoichiometric UC2 also show different types of behavior toward oxidation as a result of the absence of free carbon, which itself also changes during a stabilization treatment as a function of the parametric range and the operating conditions applied.
Generally, for the application of oxidative heat treatments, two main routes can be dissociated:                an oxidative treatment of the carbides carried out at variable temperatures (anisothermal conditions);        an oxidative treatment of the carbides applied at a fixed temperature (isothermal conditions).        
Anisothermal oxidation conditions are incompatible for the application of a stabilization process according to the present invention as they do not make it possible to guarantee stable, safe and reproducible oxidation conditions. This is because a gradual increase in the temperature applied during the treatment and consequently the introduction of energy in the form of heat into the system results in a risk of uncontrolled runaway and in unstable conditions for the oxidation of the carbides which leads to:                a sudden increase in the local temperature and in the oxidation kinetics (as illustrated in FIG. 1);        an uncontrolled runaway of the reaction and a potential spontaneous self-ignition of the UCx material (very particularly in the powder form) which is accompanied by a strong exothermic peak on the basis of an oxidation reaction enthalpy of the order of −1450 kJ/mol.        
In order to prevent these phenomena and to run a process by moderating the supply of the oxidizer starting from a predefined combustible charge and a predefined activation temperature (principle of safety of operation furthermore relevant in order to demonstrate the control of the process), an oxidative treatment under an isothermal atmosphere has to be envisaged.
Furthermore, structural and morphological differences greatly influence the behavior toward oxidation of uranium-comprising carbides, such as:                the initial nature of the material: UC has a different behavior toward oxidation than UC2 (difference in weight gain), which is also valid for UCx, rich in excess carbon;        the morphology: a powder has a substantially different ignition temperature from one or more pellets having predefined volumes and predefined densities (influence, for example, of the height of the powder bed, of the weight treated, and the like).        
The known oxidation techniques include notably several oxidation studies carried out on UC obtained by carbothermic reduction in the powder form under isothermal conditions and notably that described in the paper by Ohmichi, T. (1968), “The Oxidation of UC and UN Powder in Air”, Journal of Nuclear Science and Technology, 5, 600-602. The detailed analysis of the results shows that the data cannot be transposed to target materials of UCx type due to several constraints: a limited initial amount of material (UC weight of less than 30 mg), a range of application of oxidation temperatures which are excessively high (up to 1400° C.), in combination with a composition and with a geometric shape of the intial carbide which is different: the UC does not provide the same weight gain as the UC2 under oxidation and the geometry of the UC (powder with a particle size of 150 μm) is not representative of the UCx targets to be stabilized targeted in the present invention (comparable for the majority of them to porous centimeter-sized pellets).
Other studies carried out on bulk UC, such as those of Herrmann, “Cinétique d'oxydation du mono carbure d'uranium par l'oxygène sec ou humide [Kinetics of oxidation of uranium monocarbide by dry or humid oxygen]”, French Atomic Energy Commission Report, 19 (1968), also show profiles for variations in weight which are substantially different from those obtained with UCx target materials as a result of the difference in the initial content of the carbon in the carbide phase (increase greater than 60% in the weight gain for the formation of one and the same oxide U3O8 between the UC and the UC2 oxidized under similar conditions).
S. K. Mukerjee, G. A. R. Rao, J. V. Dehadraya, V. N. Vaidya, V. Venugopal and D. D. Sood (1994), “The Oxidation of Uranium Monocarbide Microspheres”, Journal of Nuclear Materials, 1, 97-106, and E. W. Murbach and G. E. Brand, 1965, “Pyrochemical Reprocessing of Uranium Carbide”, Summary Report, page 38, Atomics International, have also analyzed the effect of the initial weight of UC (from 30 to 200 mg for Mukerjee and up to 10 kg for Murbach) on the kinetics of oxidation. The results presented show that they cannot be transposed for the material based on uranium carbide of the present invention as the studies were not carried out under isothermal conditions (Mukerjee) and the UC samples had been initially synthesized by arc melting (Murbach), consequently exhibiting structural properties in terms of bulk density which are radically different from those of the material based on uranium carbide of the present invention.
The few facts available with regard to the oxidation of UC2 and thus the facts most representative for the targeted process vis-à-vis the structural composition of the UCx targets relate to oxidation studies carried out by Nawada et al., Thermogravimetric Study of the Oxidation Behaviour of Uranium Dicarbide, Journal of Thermal Analysis, 35 (1989), 1145-1155. The oxidative treatments brought to the UC2 were carried out under anisothermal conditions, followed by lengthy oxidation stationary states ranging from 4 to more than 100 hours. The complete oxidation cycle was consequently spread over a total time of 118 hours. The results obtained could be divided into 4 stages in order to make it easier to understand the reaction for the oxidation of the UC2 to give U3O8:
a first stage characterized by the gradual and very slow oxidation of the UC2 to give the intermediate oxide α-UO3 with a weight gain of the order of more than 19% for temperatures varying from 25 to 260° C.;
a second stage characterized by the oxidation of the carbon originating from the initial UC2 phase, which brings about a twofold weight loss, for temperatures ranging from 260 to 410° C.;
a third stage corresponding to the oxidation of the α-UO3 phase to give the oxidized U3O8 phase, which also results in a weight loss, for temperatures ranging from 410 to 560° C.;
a fourth and final stage which is defined by the oxidation of the residual free carbon, assumed to be present in the starting material, for oxidation temperatures of between 560 and 690° C., also accompanied by the recording of a weight loss.
Although this study provides data for understanding the oxidation of the UC2, it presents facts incompatible for the application of a process for the conversion of the UCx into U3O8 for several reasons:                unsuitable thermal programming conditions (mixture of anisothermal oxidation conditions, followed by lengthy oxidation stationary states) which do not satisfy the application of an oxidative heat treatment controlling the potential variations in reactivity, essential in order to guarantee the safety of the process;        an excessive oxidation time: the total duration of the oxidation of the UC2 in this study is estimated at more than 118 hours, which renders it incompatible with a semi-industrial treatment which requires the application of a process for the rapid conversion of the carbide phases into U3O8;        lack of input data, such as, for example, the initial UC2 weight (not mentioned) or the absence of physical properties of the UC2 input material (in terms of density, porosity, geometry of the pellets), which do not guarantee flexibility with regard to the oxidative treatment presented. The data provided in this study show that the material is furthermore rather different from the abovementioned UCx material (notable difference in stoichiometry of the free carbon in the initial material substantially modifying the behavior toward oxidation);        the absence of relative results related to the chemical reactivity of the UC2 during the various oxidation stages (enthalpy of each of the intermediate oxidation reactions) but also the variation in the output quantities measured (weight produced, CO2 gas produced) as a function of the input parameters (weight, O2 concentration).        
These missing facts show that this study, relevant notably for the understanding of the mechanism of oxidation of the UC2, does not make it possible to define a process as it is incompatible with the requirements of safety of a stabilization process with regard to the management of the thermal runaway and the control of the oxidation reaction by the managed introduction of an O2 partial pressure, of a controlled flow rate and of a suitable weight. Furthermore, the criteria which make it possible to guarantee the end of the reaction, apart from a total treatment at high temperature which is not compatible with the objectives/constraints related to the present invention, are not identified.
From the viewpoint of all the data existing in the bibliography, it appears that no oxidative heat treatment can be adapted to the material consisting of uranium carbide targets, having a hyperstoichiometric carbon composition, which guarantees a treatment for the conversion of the UCx to UOx by a rapid, safe and robust oxidation process corresponding to the desired functions mentioned above.